While each professor may have a different preference for what information they expect in a citation, you can use this as a general guide.
In the body of the text
Refer to the work of art using the following format: Author, Title (Figure #).
Assign figure numbers in order as each new image is first referenced in the text.
At the end of the text
Artist Name, Title of Work, Date Created.
Material or Medium, Dimensions of Work.
Repository or Owner, City or Country of Origin, Repository Accession Number.
When constructing a citation, provide what information you do know and write "unknown" for any information you cannot locate.
Joachim Wtewael, Adoration of the Shepherds, 1601.
Oil on canvas, 15.8 x 21.1 cm.
Staatsgalerie, Stuttgart, acc. 3408.